CROSS-REFERENCE TO RELATED APPLICATIONS
The present invention is related to the subject matter of the following commonly assigned co-pending applications:
Ser. No. 07/251,531 filed Sep. 30, 1988 in the name of Kevin L. Klug and entitled "Phacoemulsification Probe"; and PA1 Ser. No. 07/267,713 filed Nov. 4, 1988, which is a continuation of application Ser. No. 06/928,170 filed Nov. 6, 1986 and entitled "Control System For Ophthalmic Surgical Instruments." PA1 Application Ser. No. 428,232, filed Oct. 27, 1989 entitled "Control System For Ophthalmic Surgical Instruments"; PA1 Application Ser. No. 428,216, filed Oct. 27, 1989 entitled "Modular Cabinet For Surgical Control System"; PA1 Application Ser. No. 428,355, filed Oct. 27, 1989 entitled "Footswitch Assembly With Electrically Engaged Detents"; and PA1 Application Ser. No. 428,239, filed Oct. 27, 1989 entitled "Pneumatic Controls For Ophthalmic Surgical System".
The present invention is also related to the subject matter of the following commonly assigned applications being filed concurrently on even date herewith:
The disclosures of each and every one of the above-referenced applications is hereby incorporated herein by reference.
1. Field of the Invention
The present invention relates in general to circuits and methods for calibrating and driving ultrasonic transducers found in phacoemulsification instruments used for ophthalmic surgical procedures, and in particular to calibration methods used to determine the resonant frequency of such ultrasonic transducers and to test such transducers, and to electronic control systems for powering such ultrasonic transducers.
2. Description of Related Art
Phacoemulsification refers to the process of ultrasonic disintegration of the lens of a human or animal eye using a vibrating probe which operates at a frequency above the audio range. It is a well-known and widely used surgical procedure for disintegrating cataracts. The probe includes a hollow needle vibrating at ultrasonic frequencies to shatter the cataract; the shattered debris are withdrawn through the hollow part of the needle. The needle is mounted in an instrument which sometimes is referred to as a phacoemulsification handpiece, phaco handpiece or phaco probe. A number of designs for such handpieces or probes are known, the most common of which utilize piezoelectric transducers to produce the vibrations of the needle at ultrasonic frequencies. Aforementioned application Ser. No. 07/251,531 describes and claims one such phacoemulsification probe that is commercially available from the assignee of the present invention, namely Storz Instrument Company of St. Louis, Mo. (hereinafter "Storz").
FIGS. 1A and 1B illustrate the construction of the phaco probe disclosed in the aforementioned application Ser. No. 07/251,531. The probe 20 includes an ultrasonic transducer 22 located between a reflector 24 and resonator 26. The transducer 22 includes an electrode 30, constructed of unhardened #01 carbon steel, and two piezoelectric crystals 32 and 34. The crystals 32 and 34 may be constructed, for example, of a modified lead zirconate titanate ceramic material, formed into rings, and silver coated for electrical conductivity. Materials of this type are marketed under the trade name PXE by the Electronic Components and Materials Division of North American Phillips Corporation. An electrical lug 36 fastened to the electrode 30, allows a connection to be made to a power supply. An insulating tube 40 fits within the bore of the transducer 30. The reflector 24 is fastened to the resonator 26 by a hollow threaded tube 42 which mates to the threaded regions 44 and 46 in the reflector and resonator respectively.
Both the hollow tube 42 and the resonator 26 are preferably constructed of 6AL-4V titanium. Reflector 24 is constructed of the #17 tungsten. The insulating sleeve 40 may be made of Teflon. To assemble the components shown in FIG. 1B into the completed assembly 20 shown in FIG. 1A, threaded tube 42 is first threaded into the resonator 26 until the end 52 is seated against the shoulder 56 in the resonator 26. Then the reflector 24 is threaded onto the tube 42 until the transducer is compressed the desired amount.
The phaco probe 20 shown in FIG. 1 may be used as follows. A phacoemulsification needle 28, known in the art, such as Model No. IA-145 available from Storz, is screwed into the threaded end 58 of resonator 26. In use, the needle 28 vibrates in a longitudinal mode by alternately compressing to a retracted position illustrated by solid lines in FIG. 1A and expanding to a extended position illustrated by phantom lines 60. The vibrational displacements, indicated by dimension 62, may be anywhere from about 0.001 inch to about 0.005 inch, depending upon the strength and frequency of the electrical drive signal applied to the transducer. The vibration of the needle nominally occurs at the oscillation frequency of the piezoelectric crystals 32 and 34, which are coupled to the needle 28 through the resonator 26. Curved region 66 of the resonator 26 acts as a horn in order to impedance-match the crystals with the needle 28. Resonator 26, as a whole, functions as a one-quarter wave length transmission line (at the crystal frequency) on which needle 28 acts as a load.
The crystals 32 and 34 in FIG. 1 are driven by a signal applied to the electrode 30 and the reflector 24. The application of an alternating current signal to the crystals 32 and 34 causes them to cyclically expand to the extended position shown by phantom lines 70 in exaggerated form in FIG. 1B, and then contract to the solid position also shown in FIG. 1B. This cyclic expansion and contraction applies mechanical pulses to the resonator 26 at the signal frequency. The vibrating needle 28, when brought near a cataract, causes the cataract to shatter. The shattered debris is withdrawn through the channel 72 of the probe 20 under the influence of a vacuum generated by vacuum source 74 that is attached to the connector 76 by conventional plastic tubing represented by line 78.
As is known in the art, it is possible to construct piezoelectric crystals of various different materials, each of which has a characteristic resonant frequency. The crystal transducer 22 used with the Storz phacoemulsification probes of the type shown in FIG. 1 require a driving signal frequency applied to electrode 30 and reflector 24 in the range of 26.0 KHz to 32.0 KHz for the transducers normally used with the Storz phaco probe shown in FIG. 1. The present application will thus refer to a resonant frequency range between 26 KHz to 32 KHz, but those skilled in the art will appreciate the applicability of the present disclosure to ultrasonic transducers for medical instrument probes which operate at different frequencies.
One system for applying such driving signals to crystals 32 and 34 is described in aforementioned application. Ser. No. 07/251,531, and has been commercially used in the ophthalmic surgical console system from Storz which is sold under the trademark "DAISY."
FIG. 2 presents a block diagram of the driving circuit 80 used in the DAISY console. As is well known, the piezoelectric transducer 22 may be modeled as an RLC series resonant network in parallel with a capacitance when operating under load and near the transducer's resonant frequency. (This model of the transducer is not shown in FIG. 2.) Being a closed loop system, the driving circuit 80 is essentially an oscillator which fulfills the Barkhausen criteria for oscillation, namely it has zero phase shift and unity loop gain. The design frequency of the oscillator may be set at 28.5 KHz plus or minus 0.5 KHz for a transducer which has a nominal resonant frequency around 28.5 KHz. The feedback portion 84 of the closed loop includes an injection oscillator 88, a band pass active filter 90, low pass active filter 92, and a variable gain amplifier 94. The injection oscillator 88 provides an initial voltage signal at a frequency near the transducer resonant frequency to the injection control circuitry 96. That initial signal is disengaged from the loop of the feedback portion 84 by circuitry 96 once the driving circuit 80 provides a signal strong enough to maintain transducer oscillation. The band pass and low pass filters 90 and 92 provide the appropriate frequency selectivity and phase shifts characteristics to maintain the strength of the transducer feedback signal while the transducer phase characteristics vary over a normal operating range. The signal fed back on line 98 from the transducer 22 is derived over a compensation network 100 which provides additional frequency selectivity and phase shift stability. The variable gain amplifier 94 is used to establish the loop gain during initial calibration of the filter circuits 90 and 92, and thereafter remains fixed after the calibration is complete.
A power amplifier and transformer section 104 provides a maximum-driving voltage of about 380 volts RMS with a maximum current of about 10 mA RMS. A gain control network 106 provides a stable voltage signal output by comparing the driving voltage on line 110 with a command voltage reference level on line 112 provided by a command signal derived from a control console in accordance with the power level desired by the surgeon using the phaco probe 20, and then compensating for any differences by adjusting the gain of the power amplifier of section 104.
In operation, the closed loop portion 84 of the driving circuit 80 attempts to compensate for changes in the resonant frequency of the transducer and/or phaco probe. This changing resonance is due to a variety of local factors which materially influence the ultrasonic transducer 22 and/or probe 20. Possible factors which can vary while the probe 20 is being used include the following: (1) the degree of compression of the transducer 22 on account of changing thermal or mechanical conditions; (2) the variations in the density or other properties in the fluid and/or debris being sucked by the vacuum through the channel 72 of probe 20; (3) the mechanical pressure brought to bear against the tip of the needle 28; (4) the quality of the coupling between the resonator 26 and needle 28; and (5) changes in the efficiency of the transmission of ultrasonic energy between the crystals 32 and 34 and the resonator 26 due to minute air gaps or mechanical deformations which may occur over time. One advantage of the driving circuit 80 of FIG. 2 is that the closed loop section 84 continually attempts to make the frequency of the input signal applied to the probe 20 match the changing resonant frequency of the transducer 22 and probe 20 combination during use. Although the Q of the resonator 26 itself is very sharp, on the order of 1,000 to 2,000, and its bandwidth is very narrow, on the order of 15 to 30 Hz, the Q of the overall probe/transducer combination is much less, on the order of 40 to 100, which results in a much wider bandwidth, on the order of 300 to 750 Hz. Thus even with the closed loop system, it has proved difficult under actual conditions to achieve consistently the desired match up between the frequency of the input signal to section 104 and the actual instantaneous resonant frequency of the transducer/probe combination.
As is well known, it is beneficial, in terms of operating efficiency, to provide a driving signal to an ultrasonic transducer at its nominal resonant frequency. It has also been determined that when there is a slight mis-match between the frequency of the input signal and the natural resonant frequency of the transducer/probe combination, the stroke length of the needle is varied even as the command voltage supplied on line 112 from the console remains constant. At times, this mis-match of frequencies can result in a noticeable change in the ability of the oscillating needle 28 to perform its function, such as shattering the cataract within the eye. An operating surgeon who notices this change in performance will tend to compensate for such variations by either increasing or decreasing the strength of the command signal as needed. However, it would be quite advantageous to have a phaco probe driving circuit which is capable of maintaining the stroke length substantially constant even as the resonant frequency of the transducer/probe combination changes on account of one or more of the aforementioned five factors so that the surgeon would not have to compensate manually for such changes.
In light of the foregoing observation, it is a first object of the present invention to provide a system and method for automatically determining the resonant frequency of an ultrasonic transducer. A related object of the present invention is to provide a method for automatically checking whether an ultrasonic instrument is in proper operating condition.
A second important object of the present invention is to provide a driving system for an ultrasonic transducer/phaco probe combination which automatically maintains the needle stroke length constant for a constant input command from the user. Another related object of the present invention is to provide a method of driving an ultrasonic transducer of an ophthalmic surgical instrument by holding electrical power consumed by the transducer substantially constant for a desired level of power consumption through the use of closed loop feedback control.